Transcranial magnetic stimulation (“TMS”) uses an induction coil in which a time-varying magnetic field is generated to induce an electric field (“E-field”) within the brain. The neurons at the locations of the brain exposed to a strong enough E-field will become activated, or stimulated. In navigated brain stimulation (“NBS”), the E-field induced in the brain by a TMS induction coil device is shown as an overlay on a graphical display of an anatomical representation of the subject's brain. By viewing the display, a user can visualize the E-field induced on the brain and, therefore, interactively position the TMS coil device, in real time, in relation to the brain to stimulate a target site on the brain.
The following data acquisition and processing steps are typically performed as part of NBS.
1. A segmented data representation of the scalp or head surface of a subject is generated from data representative of the anatomical configuration of the subject's head. Typically, data representative of two-dimensional (“2D”) magnetic resonance imaging (“MRI”) images of the head of the subject, which was previously obtained using conventional MRI techniques, and where the images include at least the brain, upper parts of the skull and attached tissue and cartilage, are processed, using well known software algorithms, to generate a volumetric, three-dimensional (“3D”) representation of the head. The 3D representation of the head is then further processed, also using well known software algorithms, to generate a segmented data representation of the head surface of the subject.
2. Tracking elements are implemented to provide that the location and orientation of a TMS coil device with respect to a subject's head can be tracked. As conventional in the art, easily identifiable, reflective markers (trackers) are placed on selected points on the subject's head and also the TMS coil device to permit automatic recording of the coordinates of the points in 3D and six degrees of freedom. For example, the trackers on the TMS coil device may be a part of a tracking device attached to the TMS coil device, as described in U.S. patent application for TRANSCRANIAL MAGNETIC STIMULATION INDUCTION COIL DEVICE WITH ATTACHMENT PORTION FOR RECEIVING TRACKING DEVICE, Ser. No. 11/847,544 filed Aug. 30, 2007, assigned to the assignee of this application and incorporated by reference herein, and the coordinates of the trackers are recorded using a special-purpose camera, as conventional in the art.
3. A co-registration procedure is performed, which correlates data representative of the coordinates of the trackers on the TMS coil device and the subject's head (2. above) with the image data from which the 3D representation of the subject's head is generated (1. above). Typically, several landmark points on the head, such as points on each ear and the nose, are pinpointed from the 2D MRI images or, if available, the volumetric 3D image of the head. The same points are also pinpointed on the subject's head by use of a digitization pen tracker. After performing such point-to-point correspondences or point-to-point matching, a transformation is computed that aligns the coordinate system of the MRI images of the head with the coordinate system of the trackers. The quality of the transformation can be enhanced, for example, at least in the least-squares sense, by performing additional point-to-point matching, which in turn improves the accuracy of NBS.
4. On a display typically used in NBS, a graphical representation of the TMS coil device, in particular preferably only the casing of the TMS coil device in which the coil windings are contained, is shown in relation to a graphical representation of the scalp and a portion of the brain at a selected depth, and the E-field induced on the brain portion by the TMS coil device is shown as an overlay on the brain portion. The display provides a user with a visual representation of the position and orientation of the casing, and thus the coil windings, of the TMS coil device in relation to the head and the brain, and also the E-field induced in the brain, as the user navigates the TMS coil device in relation to the subject's head. The quality of the transformation computed in the co-registration (3. above) affects the accuracy of the representations shown on the display and, thus, the navigation accuracy. As well known in the art, the E-field induced by the coil windings is computed using a head shape model, e.g., a spherical model, such as described in Ravazzani, P., et al., “Magnetic stimulation of the nervous system: induced electric field in unbounded, semi-infinite, spherical, and cylindrical media,” Annals of Biomedical Engineering 24: 606-616, 1996, incorporated by reference herein, and based on a model of the shape and location of the copper windings inside the casing of the TMS coil device. The E-field is then shown on the representation of the brain portion, for example, using colors to indicate E-field strength, to provide that the user can navigate the TMS coil device to stimulate target sites on the brain portion. The accuracy of the representation of the brain portion, in a large part, determines the accuracy of the representation of the E-field induced on the brain portion shown on the display and, thus, greatly impacts the accuracy with which the user can navigate the TMS coil device to stimulate target sites on the brain.
It is known that the effects of TMS depend on both the absolute strength of the E-field at a target site on the brain and the relative strength of the E-field with respect to regions neighboring the target site. Consequently, it is important that the NBS display accurately show to the user of a TMS coil device the location of the maximum E-field in a neighborhood of interest for a target site. For anatomical reasons, the relevant neighborhoods of interest on a brain portion of a selected depth usually are represented on a prior art NBS display as surfaces oriented approximately parallel to the portion of the subject's scalp above the neighborhoods. Hence, in prior art NBS, the brain has been represented using a plurality of so-called visualization surfaces, each of which represents a portion of the brain at a selected depth.
In the prior art, an NBS display usually shows a visualization surface at a depth of about 20 to 25 mm beneath the scalp. The visualization surface approximates the shape of the brain and the cortical brain structures at the selected depth. In addition, the NBS display shows the E-field induced at points along the visualization surface, including the location of the maximum E-field on the visualization surface. The E-field ordinarily is shown as a colored map, where the coloration indicates strength relative to a maximum. In addition, the NBS display typically shows the TMS coil device in color in relation to the visualization surface. In addition, the visualization surface is also shown using colored and textured surfaces of polygons that are updated in real time, for example, as the TMS coil device or the head moves. Further, the prior art NBS display provides that the TMS coil device and the visualization surfaces may be viewed from any angle and distance.
In prior art NBS, a visualization surface is derived directly from the 2D MRI image data of the head, such that any protrusions (bumps), concavities or other irregularities in the scalp are correspondingly, substantially identically reflected in the visualization surface. Ideally, it is expected that the location of the maximum E-field would move along the visualization surface on an NBS display in correspondence with the movement of the TMS coil device along the scalp.
It is has been observed, however, on prior art NBS displays showing visualization surfaces, which have significant bumps, protrusions or irregularities, in relation to a TMS coil device and the E-field induced by the TMS coil device, that when the TMS coil device is moved gradually along the scalp of a subject, the representation of the E-field on the visualization surface becomes irregular and does not directly correspond to the movement of the TMS coil device along the scalp. For example, if a stimulation target site is at the bottom of a surface concavity on the visualization surface, the strength of the E-field at the target site displayed on the visualization surface is significantly weaker than that at points surrounding the target site, even though the actual strength of the E-field at the target site may be the same or about the same as the strength of the E-field at the surrounding points. It is well known in the art that the E-field induced by a TMS coil device weakens quickly with distance from the coil windings within the TMS coil device, and for example may weaken even about 5-20% at a distance of about 2 mm from the coil windings. Therefore, if a visualization surface closely approximates the shape of the scalp, the maximum E-field likely will not be shown as being located in a concavity in the visualization surface which constitute a stimulation target site and, instead, will be shown as being located at points on the visualization surface neighboring the target site and bordering the concavity. Consequently, where the visualization surface includes concavities that correspond to target sites, it is very difficult, if not impossible, to position the TMS coil device so that the maximum E-field is located at the target site on the visualization surface.
Alternatively, if a visualization surface does not closely approximate the shape of the brain, then the regions neighboring a target site likely will not be properly oriented, such that the objective of using NBS to accurately position the TMS coil device in relation to the head, so as to induce a maximum E-field at a target site, becomes unattainable.
Therefore, there exists a need for generating a visualization surface representative of a portion of the brain at a selected depth, for use in displaying the E-field induced on the brain by a TMS coil device as part of NBS, which accurately represents the brain portion at the selected depth and avoids erroneous representations of the E-field on the visualization surface.